Engines for Solids

The biggest excitement in the missile-preoccupied Pentagon is the rapid progress of solid-propellant rockets. Not only the solid fuels have been improved. So have the engines that were once full of faults. The details of these improvements are still secret, but they can be described in general terms.

A potential troublespot in any rocket engine is the nozzle through which the hot gases escape into the air. Liquid fuels can be used to cool the nozzle, circulating through its hollow walls or seeping through small holes to provide a protective layer on its inner surface. Solid fuel cannot do this, but other means have been developed to keep the racing gases from destroying the nozzle. It is lined with some such high-melting-point material as graphite or zirconium oxide. As the fuel burns, the nozzle enlarges somewhat because of erosion, but the burning rate of the fuel is planned so that the pressure of the gases remains where it should be.

Load into Speed. Liquid-fuel rockets burn their fuel only as fast as their pumps, which must be kept light, can deliver it to the combustion chamber. This limitation keeps the thrust comparatively low, and low thrust means a long burning time. Thus, a heavy load of fuel is carried to high altitude against the pull of gravitation before it is burned and its energy turned into speed.

Solid-fuel rockets avoid most of this waste of energy by burning their fuel very fast--in a few seconds, if desirable. Instead of struggling painfully off the ground as liquid-fuel rockets do, the solid-fuel bird can be gone in a flash. Its higher speed while still in the dense lower atmosphere costs something in aerodynamic drag, but since solid-fuel rockets have no pumps, valves or plumbing, they are more compact and can slip through the air more easily.

All the fuel of a solid-fuel rocket is in a single, roughly cylindrical container, and when the fuel burns, the container's walls are subjected to high pressure. They must be strong but also light, and one of the most promising materials is sheet plastic reinforced with glass fibers. Neither plastic nor glass is heat-resistant, but they do not need to be. The fuel burns from the center outward, and the unburned portion of it protects the wall from heat until nearly all the fuel is gone.

Control by Blow-Off. To hit a distant target accurately, a long-range ballistic missile must be steered in the right direction and must attain the right speed. If it is traveling 23,000 ft. per sec. (15,600 m.p.h.), an error of I ft. per sec. in its top speed will make it miss its target by 500 yds. So when the desired speed has been reached, the thrust must be cut off accurately in a small fraction of a second. This is not too difficult with liquid-fuel rockets, whose thrust can be cut by shutting off the fuel. Solid-fuel rockets cannot be controlled in this simple way, but other effective ways have been developed. One of them is to blow off part of the nozzle, or the pressure-wall, when the right speed has been reached.

The pressure inside the rocket falls abruptly. The fuel stops burning, and the thrust drops to zero. If this kind of cutoff is not accurate enough, small vernier rockets can be used to give the proper amount of extra push. Or retrorockets thrusting in reverse can shave a few feet per second off the rocket's speed.

Solid-fuel rockets still have their weaknesses. They must be shipped pretty much in one piece, which is difficult in the case of very large birds. Their fuel has improved enormously in the last few years, and will surely improve still more, but it still does not have the propulsive force of the best liquid fuels.

Offsetting these disadvantages are the simplicity and ruggedness of the solid-fuel rockets. They have no pipes or valves to leak, no pumps to malfunction. Unlike their liquid-fuel rivals, which take hours to check and fill, a solid-fuel rocket is always ready to go. Set on the launching pad, it is always ready to have its fire lighted.

Both the Army and Navy, furiously at work on solid-fuel rockets, are smug about their achievements and prospects. Even the Air Force, which is deeply committed to liquid-fuel rockets (Atlas, Thor, Titan), is going in for solids.

One specific Air Force proposal is not to develop liquid-propelled missiles for operational use after 1962. Liquid-fuel rockets may still have a future in space travel, but many Pentagon experts believe that their military cutoff point is in sight.

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